Browsing by Author "Lupetin, Piero"

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Charge carrier defect chemistry of nanoscopic SrTiO3
(2012) Lupetin, Piero; Maier, Joachim (Prof. Dr.)
The study of ionic and electronic conduction properties of nanosized objects has revealed, in the last years, a variety of fascinating effects as the conduction properties of nanocrystalline materials are dominated if not fully controlled by the grain boundaries. The basis for the understanding of such effects is provided by the field of nanoionics, which allows the elucidation of defect chemistry not only for well separated boundary zones but also in the more exciting mesoscopic range where the distance of the interfaces (grain size) is on the order or below the characteristic decay length of a semi-infinite interface. In the present study, strontium titanate (SrTiO3) has been taken as a model system to investigate these aspects. Notably, SrTiO3 is an excellent example for electroceramic oxides in general and for the family of perovskites in particular, thanks to its pronounced stability and its well studied defect chemistry at the macroscale. Furthermore, it exhibits a great technological relevance for several different applications such as anode for solid oxides fuel cells, varistors as well as substrate for high temperature superconductors. In the field of solid state ionics its importance is due to the fact that it is a mixed ionic and electronic conductor, with characteristic variations in the typical window of experimental conditions. In this contribution the electrical properties of SrTiO3 are investigated at the nanoscale, when no unperturbed bulk is present and the overall electrical properties are clearly dominated by the grain boundaries. Acceptor (iron) as well as donor (niobium) doping has been used to adjust the properties of the material (conductivity, space charge potential) even in the mesoscopic regime. In order to investigate size effects on the conduction properties the preparation of nanostructured SrTiO3 with a grain size smaller than 100 nm comes to the fore. This implies the optimization of the synthesis procedure at low temperature as well as of the sintering process. For the synthesis procedure three different methods are considered, namely co-precipitation, combustion and solvothermal and the densification was carried out using the high pressure field assisted sintering, also known as spark plasma sintering. Once the preparation of the nanostructured material is achieved, the attention is focused on the characterization of the electrical properties. In this context, the oxygen non-stoichiometry is considered as a key element, since it plays a crucial role in determining whether SrTiO3 is a p-type, n-type or ionic conductor. Therefore, the conduction properties have been investigated over a broad range of oxygen partial pressures and temperatures by means of impedance spectroscopy. In the case of undoped SrTiO3 (characterized by intrinsic acceptor impurities), the stoichiometry variation of the mesoscopic situation, in which the space charge zones overlap, reveals several exciting size-induced phenomena such as: increase of n-type conductivity by several orders of magnitude, an equally great depression of p-type conductivity and an even stronger drop of the oxygen vacancy conductivity when compared to the macroscopic situation. This generates a giant shift of the conductivity minimum by several orders of magnitude in terms of partial pressure. The results can be explained in the light of space charge effects occurring as a consequence of a positive charge excess in the grain boundary core. Huge size effects are observed also in intentionally acceptor doped nanocrystalline SrTiO3 and the difference with respect to the nominally pure case can be explained by the higher doping level. Another aspect, considered in this study concerns the possibility of tuning the grain boundary properties. This point is particularly relevant in mesoscopic materials, in which the grain boundaries control the overall charge transport. This goal is achieved by adding the dopant at the grain boundaries in order to modify only locally the stoichiometry. In particular, in SrTiO3 it is observed that the addition of acceptors, namely iron, at the grain boundaries yields to a core-shell situation within the grain in which the highly conductive shell short-circuits the bulk and determines the overall conduction properties of the material. Particularly intriguing are the studies on donor (niobium) doped SrTiO3, which is a well known n-type conductor at relatively high temperatures in the high oxygen partial pressure range. Surprisingly, the nanocrystalline material showed p-type conductivity in oxidizing conditions at 550°C and a blocking effect of the grain boundaries with respect of the electron transport when the material switches to the n-type regime. The whole set of results make nanocrystalline SrTiO3 a formidable master example of defect chemistry in the nanocrystalline regime and demonstrate the enormous power of size as degree of freedom in modern materials research.